puns involving substitution of meaning

Hi,
I'm trying to come up with a birthday card based on variations of punning phrases. For example, 'How long is a piece of string?' becomes 'How long is a birthday?' The answer will be 'Longer than a piece of string.' There's a substitution involved in making the pun.

Similarly, the pun 'Time flies like bananas, fruit flies like an arrow' involves substitution. This pun works because it confuses one meaning of a word with another. I'm trying to think of other words that I can do this with, to create a birthday card that's a series of questions (How long, how far,...) whose answers involve play on those words.

Any suggestions?

Perhaps psychiatric evaluation..

They don't work, do they?

Try 'the penis mightier than the sword' - at least a knob-gag will usually get a laugh.

Here are three sources you might be able to use;
A fat sportsman said, "Call me anything you like, but don't call me late for dinner."
Groucho Marks said, "If you can't leave in a taxi, you can leave in a huff. If you can't leave in a huff, you can leave in a minute and a huff."
From an earlier thread on this site, "She left in a Chevrolet and a huff."